US20030169986A1 - Optical fiber, optical component, dispersion compensating fiber module, and optical communication system - Google Patents

Abstract

The present invention relates to a dispersion compensating fiber module excellent in pumping efficiency. The dispersion compensating fiber module includes a dispersion compensating optical fiber for compensating for a chromatic dispersion of an optical transmission line through which signal light propagates, and an optical fiber that can function as a Raman amplification optical fiber. The optical fiber that can function as a Raman amplification optical fiber, in particular, has the characteristics including an FOM Raman of 8 (1/dB/W) or more with respect to signal light having a wavelength of 1,650 nm when pumping light having a wavelength of 1,550 nm is supplied, and a chromatic dispersion with an absolute value of 10 ps/nm/km to 50 ps/nm/km at the wavelength of 1,650 nm.

Description

BACKGROUND OF THE INVENTION
• 1. Field of the Invention[0001]
• The present invention relates to an optical fiber suitable for a Raman amplification medium, an optical component including the optical fiber, a dispersion compensating fiber module which can compensate for a chromatic dispersion in an optical transmission line through which signal light is transmitted and can also perform Raman amplification using the optical fiber, and an optical communication system including the dispersion compensating fiber module.[0002]
• 2. Related Background Art[0003]
• An optical communication system performs WDM (Wavelength Division Multiplexing) optical communication of transmitting/receiving large-volume information at high speed by transmitting signal light of a plurality of channels having different wavelengths to an optical transmission line. In general, a single-mode optical fiber used as an optical transmission line has a zero dispersion wavelength near a wavelength of 1.3 μm and a relatively large positive chromatic dispersion in a signal wavelength band such as the C-band (1,530 nm to 1,565 nm) or L-band (1,565 nm to 1,625 nm). When signal light propagates through an optical transmission line having such a chromatic dispersion, the transmission quality deteriorates due to a deterioration in signal waveform, resulting in difficulty in large-volume communication.[0004]
• In order to realize a large-capacity optical communication system, the absolute value of the cumulative chromatic dispersion in a signal light propagation path must be reduced. More specifically, a dispersion compensating fiber module is used, which has a chromatic dispersion whose sign is different from that of a chromatic dispersion in an optical transmission line in the signal wavelength band. In this case, the absolute value of the total cumulative chromatic dispersion of the optical transmission line and dispersion compensating fiber module becomes smaller than that of the cumulative chromatic dispersion in only the optical transmission line. This suppresses a deterioration in signal waveform and improves the transmission quality. Although such dispersion compensating fiber modules exist in various forms, a form including a dispersion compensating optical fiber is widely known. When a dispersion compensating optical fiber is applied, the absolute value of the total cumulative chromatic dispersion of the optical transmission line and dispersion compensating optical fiber is brought close to zero by properly setting the length of the dispersion compensating optical fiber.[0005]
• In addition, when a single-mode optical fiber having a positive chromatic dispersion in the signal wavelength band is used as an optical transmission line, a dispersion compensating optical fiber having a negative chromatic dispersion in the signal wavelength band is used. In this case, since the dispersion compensating optical fiber generally has a small effective area, a nonlinear optical phenomenon tends to occur. For this reason, this dispersion compensating optical fiber can also function as a Raman amplification optical fiber which Raman-amplifies signal light upon reception of Raman amplification pumping light. That is, the loss intrinsic to the dispersion compensating optical fiber is compensated for by a Raman amplification gain, and the dispersion compensating optical fiber is reduced in effective loss to zero or has an effective gain.[0006]
• A dispersion compensating fiber module having both the dispersion compensating function and the Raman amplification function is disclosed in, for example, Japanese Patent Laid-Open No. 11-174504. In the dispersion compensating fiber module disclosed in this reference, dispersion compensation and Raman amplification of signal light in a dispersion compensating optical fiber having a chromatic dispersion of −50 ps/nm/km or less at a signal wavelength are realized by supplying pumping light to the dispersion compensating optical fiber.[0007]
SUMMARY OF THE INVENTION
• From a study of the above conventional technique, the present inventor found the following problem. The dispersion compensating fiber module disclosed in Japanese Patent Laid-Open No. 11-174504 has low pumping efficiency in Raman amplification, and hence requires a larger power of pumping light to obtain a required gain.[0008]
• The present invention has been made to solve the above problem, and has as its object to provide a dispersion compensating fiber module excellent in pumping efficiency, an optical fiber that can be suitably used as a signal light propagation path in this dispersion compensating fiber module, an optical component including the optical fiber, and an optical communication system including the dispersion compensating fiber module.[0009]
• An optical fiber according to the present invention can function as a Raman amplification optical fiber and can be applied to a dispersion compensating fiber module. This optical fiber has an FOM Raman of 8 (1/dB/W) or more with respect to signal light having a wavelength of 1,650 nm when pumping light having a wavelength of 1,550 nm is supplied, and a chromatic dispersion of 10 ps/nm/km to 50 ps/nm/km in absolute value at a wavelength of 1,650 nm.[0010]
• The optical fiber according to the present invention preferably has an effective area of 15 μm[0011]²or less with respect to light having a wavelength of 1,550 nm. In addition, the optical fiber according to the present invention preferably has a Raman gain coefficient of 0.005 (1/Wm) or more and a nonlinear coefficient of 20 (1/W/km) or more.
• In order to facilitate adjustment of the dispersion value of a dispersion compensating fiber module to which an optical fiber according to the present invention can be applied, the absolute value of the chromatic dispersion at a wavelength of 1,650 nm is preferably 15 ps/nm/km to 40 ps/nm/km, and more preferably 20 ps/nm/km to 30 ps/nm/km. In addition, a dispersion slope at a center wavelength in a signal wavelength band is preferably −0.3 ps/nm[0012]²/km to +0.1 ps/nm²/km.
• The optical fiber having the above structure can be applied to a dispersion compensating fiber module, and such a dispersion module (a dispersion compensating fiber module according to the present invention) includes the above optical fiber (an optical fiber according to the present invention) and a dispersion compensating optical fiber having a chromatic dispersion whose sign is opposite to that of the chromatic dispersion of the optical transmission line in a signal wavelength band.[0013]
• An optical communication system according to the present invention includes an optical transmission line (including a repeating transmission line disposed between repeaters) for transmitting signal light, which is laid between an optical transmitter and an optical receiver, and the above dispersion compensating fiber module.[0014]
• Note that when the optical fiber according to the present invention is applied to an optical component, it can be used in various fields including a Raman amplifier, a dispersion compensating fiber module, and the like. In this case, this optical component (an optical component according to the present invention) includes the optical fiber, a first optical connector attached to one end of the optical fiber, an optical isolator having one end optically connected to the other end of the optical fiber, and a second optical connector attached to the other end of the optical isolator. In order to realize miniaturization, the optical component may include a case which houses the optical fiber and optical isolator.[0015]
• A Raman amplifier (an optical amplifier according to the present invention) to which the optical fiber is applied includes a supply unit for supplying pumping light to the optical fiber, in addition to the optical fiber. Note that the Raman amplifier may further include an optical isolator optically connected to one end of the optical fiber.[0016]
• When Raman amplification is to be performed, there are various forms of supplying pumping light (pumping light of one or more channels having different wavelengths) from the supply unit. More specifically, when backward pumping is to be performed in the Raman amplifier, the supply unit includes a structure for supplying, to the optical fiber prepared as a Raman amplification optical fiber, pumping light for Raman amplification which propagates in a direction opposite to the propagating direction of signal light (signal light of one or more channels having different wavelengths). When forward pumping is to be performed, the supply unit includes a structure for supplying, to the optical fiber prepared as a Raman amplification optical fiber, pumping light for Raman amplification which propagates in the same direction as the propagating direction of signal light. When bidirectional pumping is to be performed, the supply unit includes a structure for supplying, to the optical fiber prepared as a Raman amplification optical fiber, pumping light for Raman amplification which propagates in a direction opposite to the propagating direction of signal light, and also supplying, to the optical fiber, pumping light for Raman amplification which propagates in the same direction as the propagating direction of signal light.[0017]
• More specifically, the dispersion compensating fiber module according to the present invention is a dispersion compensating fiber module for compensating for a chromatic dispersion in an optical transmission line through which signal light propagates, and includes an input terminal, output terminal, dispersion compensating optical fiber, and Raman amplification optical fiber (an optical fiber according to the present invention which has the above structure). The above input terminal is prepared to receive signal light (WDM signal light) of one or more channels having different wavelengths. The above output terminal is prepared to output signal light. The above dispersion compensating optical fiber is disposed on an optical path between the input terminal and the output terminal, and has a chromatic dispersion whose sign is different from that of a chromatic dispersion in an optical transmission line in a signal wavelength band. The above Raman amplification optical fiber is disposed on the optical path between the input terminal and the output terminal, and has an FOM Raman of 8 (1/dB/W) or more with respect to signal light having the wavelength of 1,650 nm when pumping light having the wavelength of 1,550 nm is supplied, and a chromatic dispersion which is ½ or less that of the dispersion compensating optical fiber at the wavelength of 1,650 nm and 10 ps/nm/km to 50 ps/nm/km in absolute value.[0018]
• Note that above Raman amplification optical fiber may be disposed between the dispersion compensating optical fiber and the output terminal. When backward pumping is to be performed in this arrangement in particular, if an optical isolator is disposed between the dispersion compensating optical fiber and the Raman amplification optical fiber, signal light passing through the dispersion compensating optical fiber is guided to the Raman amplification optical fiber via the optical isolator. On the other hand, pumping light passing through the Raman amplification optical fiber and reflected light components produced in the Raman amplification optical fiber are blocked by the optical isolator. In addition, the Raman amplification optical fiber may be disposed between the input terminal and the dispersion compensating optical fiber. The above dispersion compensating optical fiber may include first and second dispersion compensating optical fibers, and the optical fiber may be disposed between the first and second dispersion compensating optical fibers.[0019]
• A dispersion compensating fiber module according to the present invention includes at least a dispersion compensating optical fiber and a case which houses the dispersion compensating optical fiber, and may further include a structure which allows a constituent element to be easily added or omitted. The above case includes a first connector for receiving signal light from an optical transmission line, and a second connector for sending out signal light to the optical transmission line. One end of the dispersion compensating optical fiber housed in the case is optically coupled to the first connector, and a third connector having a structure that can be coupled to the second connector is attached to the other end of the dispersion compensating optical fiber.[0020]
• With the above structure, when Raman amplification is not performed in the dispersion compensating fiber module, the third connector attached to the other end of the dispersion compensating optical fiber may be coupled to the second connector of the case. When Raman amplification is to be performed in the dispersion compensating fiber module, a Raman amplification optical fiber having fourth and fifth connectors attached to its two ends is prepared, and the fourth connector attached to one end of the Raman amplification optical fiber is coupled to the third connector attached to the other end of the dispersion compensating optical fiber. In addition, the fifth connector attached to the other end of the Raman amplification optical fiber is coupled to the second connector of the case, thereby allowing the Raman amplification optical fiber to be easily disposed between the dispersion compensating optical fiber and the output terminal (corresponding to the second connector) of the dispersion compensating fiber module.[0021]
• Assume that both the Raman amplification optical fiber and the dispersion compensating optical fiber are to be housed in the case of the dispersion compensating fiber module. In this case, in order to miniaturize the dispersion compensating fiber module, it is preferable that the Raman amplification optical fiber be wound in the form of a coil first, and then the dispersion compensating optical fiber be wound in the form of a coil around the Raman amplification optical fiber wound in the form of a coil. This is because the Raman amplification optical fiber having a high nonlinear coefficient is more resistant to a bending loss than the dispersion compensating optical fiber, and hence can be wound into a smaller diameter.[0022]
• An optical communication system according to the present invention may include an optical transmission line for transmitting signal light and the above dispersion compensating fiber module having a structure which can realize different connection forms depending on whether Raman amplification is to be done or not.[0023]
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a view showing the schematic arrangement of an optical communication system according to the present invention;[0024]
• FIG. 2 is a view showing the arrangement of the first embodiment of a dispersion compensating fiber module according to the present invention;[0025]
• FIG. 3 is a view showing the arrangement of the second embodiment of the dispersion compensating fiber module according to the present invention;[0026]
• FIG. 4 is a view showing the arrangement of the third embodiment of the dispersion compensating fiber module according to the present invention;[0027]
• FIGS. 5A and 5B are views showing the arrangement of the first embodiment of a fiber module that can be applied to the dispersion compensating fiber module according to the present invention;[0028]
• FIG. 6 is a view showing the arrangement of the second embodiment of the fiber module that can be applied to the dispersion compensating fiber module according to the present invention;[0029]
• FIG. 7 is a view showing the arrangement of the third embodiment of the fiber module that can be applied to the dispersion compensating fiber module according to the present invention;[0030]
• FIGS. 8A and 8B are a sectional view showing the structure of an optical fiber according to the present invention and its refractive index profile;[0031]
• FIG. 9A is a perspective view for explaining a mounted state of a dispersion compensating optical fiber in the dispersion compensating fiber module according to the present invention, and FIG. 9B is a perspective view for explaining a mounted state of an optical component according to the present invention which can be applied to the dispersion compensating fiber module shown in FIG. 9A;[0032]
• FIGS. 10A and 10B are a plan view and sectional view for explaining a mounted state of a dispersion compensating optical fiber and Raman amplification optical fiber (included in optical fibers according to the present invention);[0033]
• FIG. 11 is a table showing specifications of dispersion compensating optical fibers and Raman amplification optical fibers which can be applied to the dispersion compensating fiber module according to the present invention;[0034]
• FIG. 12 is a graph showing the relationship between the required power of pumping light and the length of an optical transmission line in embodiment samples and comparative example samples of dispersion compensating fiber modules;[0035]
• FIG. 13 is a graph showing the relationship between the required recovery amount of pumping light power and the length of an optical transmission line in dispersion compensating fiber modules according to embodiment samples with respect to a comparative example sample;[0036]
• FIG. 14 is a graph showing noise figure characteristics (NF) in forward pumping (pumping in the forward direction) and backward pumping (pumping in the backward direction);[0037]
• FIG. 15 is a graph showing relative intensity noise characteristics (RIN) in forward pumping (pumping in the forward direction) and backward pumping (pumping in the backward direction);[0038]
• FIG. 16 is a graph showing the relationship between the power of pumping light and the effective gain in the dispersion compensating fiber module; and[0039]
• FIG. 17 is a graph showing the relationship between the noise figure and the effective gain in the dispersion compensating fiber module.[0040]
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
• Each embodiment of an optical fiber, optical component, dispersion compensating fiber module, and optical communication system according to the present invention will be described in detail below with reference to FIGS.[0041]1 to4, FIGS. 5A and 5B, FIGS. 6 and 7, FIGS. 8A to10B, and FIGS.11 to17. The same reference numerals denote the same parts throughout the drawings, and a repetitive description thereof will be avoided.
• FIG. 1 is a view showing the schematic arrangement of an optical communication system according to the present invention. An[0042]optical communication system1 includes anoptical transmitter10,optical repeater20, andoptical receiver30. Anoptical transmission line40 is laid between theoptical transmitter10 and theoptical repeater20. Anoptical transmission line50 is laid between theoptical repeater20 and theoptical receiver30. Theoptical repeater20 incorporates a dispersion compensatingfiber module21 andoptical amplifier22 according to the present invention.
• The[0043]optical transmitter10 sends out signal light (multiplexed signal light) of a plurality of channels having different wavelengths to theoptical transmission line40. The dispersion compensatingfiber module21 in theoptical repeater20 receives the signal light having reached via theoptical transmission line40. The dispersion compensatingfiber module21 then performs dispersion compensation of the signal light and Raman-amplifies it. Theoptical amplifier22 includes an optical fiber doped with a rare-earth element (e.g., an Er element) as an optical amplification medium. The signal light output from the dispersion compensatingfiber module21 is amplified in the optical amplification medium by supplying pumping light for pumping the rare-earth element to the optical amplification medium. The amplified signal light is sent out to theoptical transmission line50. Theoptical receiver30 receives the multiplexed signal light having reached through theoptical transmission line50, and demultiplexes the multiplexed signal light into light components of the respective channels. After demultiplexing, light of each channel is received by theoptical receiver30. Each of theoptical transmission lines40 and50 is formed from a standard silica-based single-mode optical fiber having a zero dispersion wavelength near a wavelength of 1.3 μm.
• The arrangement of the dispersion compensating[0044]fiber module21 included in theoptical communication system1 will be described next with reference to FIGS.2 to4.
• FIG. 2 is a view showing the arrangement of the first embodiment of the dispersion compensating fiber module according to the present invention. A dispersion compensating[0045]fiber module21A shown in FIG. 2 can be applied to the dispersion compensatingfiber module21 in FIG. 1. The dispersion compensatingfiber module21A has anoptical isolator211,fiber unit230,optical coupler222, andoptical isolator212 sequentially arranged along a signal light propagation path extending from aninput terminal201A to anoutput terminal202A. The dispersion compensatingfiber module21A also includes a pumping light source242 (pumping light supply unit) connected to theoptical coupler222. The pumpinglight source242 andoptical coupler222 implement a structure for supplying pumping light to thefiber unit230 in the backward direction (backward pumping).
• The[0046]optical isolators211 and212 transmit light in the signal light propagating direction from theinput terminal201A to theoutput terminal202A but do not transmit light in the backward direction. The pumpinglight source242 outputs pumping light for Raman amplification, and, for example, a semiconductor laser source is suitably used as the pumpinglight source242. Theoptical coupler222 outputs the pumping light output from the pumpinglight source242 to thefiber unit230, and also outputs the signal light output from thefiber unit230 to theoptical isolator212.
• The[0047]fiber unit230 includes a dispersion compensating optical fiber having a chromatic dispersion whose sign is opposite to that of the chromatic dispersion of the optical transmission line in a signal light wavelength band including a wavelength of 1,650 nm, and an optical fiber optically connected to this dispersion compensating optical fiber. This optical fiber can be used as a Raman amplification optical fiber and preferably has an FOM Raman of 8 (1/dB/W) or more with respect to signal light having a wavelength of 1,650 nm when pumping light having a wavelength of 1,550 nm is supplied, and a chromatic dispersion of 10 ps/nm/km to 50 ps/nm/km in absolute value, preferably 15 ps/nm/km to 40 ps/nm/km in absolute value, and more preferably 20 ps/nm/km to 30 ps/nm/km in absolute value.
• The[0048]fiber unit230 performs Raman amplification and dispersion compensation of signal light when the pumping light output from the pumpinglight source242 is supplied to thefiber unit230. Note that when the Raman amplification optical fiber is made of silica glass as a main component, the pumping light wavelength is set to be shorter than the signal light wavelength by 100 nm.
• In the dispersion compensating[0049]fiber module21A according to the first embodiment, the pumping light for Raman amplification which is output from the pumpinglight source242 is supplied to propagate to thefiber unit230 in a direction opposite to the signal light propagating direction via theoptical coupler222. On the other hand, the signal light input from theinput terminal201A is subjected to dispersion compensation and Raman amplification in thefiber unit230. The resultant light is output from theoutput terminal202A via theoptical coupler222 andoptical isolator212. Such backward pumping is preferable in that the Raman amplification characteristics are excellent with little influence of intensity noise of pumping light.
• FIG. 3 is a view showing the arrangement of the second embodiment of the dispersion compensating fiber module according to the present invention. A dispersion compensating[0050]fiber module21B shown in FIG. 3 can also be applied as the dispersion compensatingfiber module21 in FIG. 1. This dispersion compensatingfiber module21B has anoptical isolator211,optical coupler221,fiber unit230, andoptical isolator212 sequentially arranged along a signal light propagation path extending from aninput terminal201B to anoutput terminal202B. The dispersion compensatingfiber module21B also includes a pumping light source241 (supply unit) connected to theoptical coupler221. The pumpinglight source241 andoptical coupler221 supply pumping light to thefiber unit230 in the forward direction (forward pumping).
• The pumping[0051]light source241 outputs pumping light for Raman amplification, and, for example, a semiconductor laser source is suitably used as the pumpinglight source241. Theoptical coupler221 outputs the pumping light output from the pumpinglight source241 to thefiber unit230, and also outputs the signal light output from theoptical isolator211 to thefiber unit230. Thefiber unit230 performs Raman amplification and dispersion compensation of signal light when the pumping light output from the pumpinglight source241 is supplied to thefiber unit230.
• In this dispersion compensating[0052]fiber module21B, the Raman amplification pumping light output from the pumpinglight source241 is supplied to propagate to thefiber unit230 in the same direction as the signal light propagating direction via theoptical coupler221. On the other hand, the signal light input from theinput terminal201B is input to thefiber unit230 via theoptical isolator211 andoptical coupler221 and subjected to dispersion compensation and Raman amplification in thefiber unit230. The resultant light is output from theoutput terminal202B via theoptical isolator212. Such forward pumping is preferable in that an increase in noise figure is suppressed.
• FIG. 4 is a view showing the arrangement of the third embodiment of the dispersion compensating fiber module according to the present invention. A dispersion compensating[0053]fiber module21C shown in FIG. 4 can also be used as the dispersion compensatingfiber module21 in FIG. 1. This dispersion compensatingfiber module21C has anoptical isolator211,optical coupler221,fiber unit230,optical coupler222, andoptical isolator212 sequentially arranged along the signal light propagation path extending from aninput terminal201C to anoutput terminal202C. The dispersion compensatingfiber module21C also includes a pumpinglight source241 connected to theoptical coupler221, and a pumpinglight source242 connected to theoptical coupler222. In the arrangement shown in FIG. 4, pumping light is supplied to thefiber unit230 in both the directions, i.e., the forward and backward directions (bidirectional pumping).
• In this dispersion compensating[0054]fiber module21C, the Raman amplification pumping light output from the pumpinglight source241 is supplied to propagate to thefiber unit230 in the same direction as the signal light propagating direction via theoptical coupler221. The Raman amplification pumping light output from the pumpinglight source242 is supplied to propagate to thefiber unit230 in a direction opposite to the signal light propagating direction via theoptical coupler222. The signal light input from theinput terminal201C is input to thefiber unit230 via theoptical isolator211 andoptical coupler221 and subjected to dispersion compensation and Raman amplification in thefiber unit230. The resultant light is output from theoutput terminal202C via theoptical coupler222 andoptical isolator212. Such bidirectional pumping is preferable in that the influence of intensity noise of pumping light is small and an increase in noise figure is suppressed.
• In each of the arrangements shown in FIGS.[0055]2 to4, the installation of theoptical isolator211 on the input terminal side prevents the propagation of pumping light and unnecessary scattered light from the input terminal to the upstream side. In addition, the installation of theoptical isolator212 on the output terminal side prevents the propagation of noise light from the output terminal to the downstream side. Each of theoptical couplers221 and222 may have an arrangement including a dielectric multilayer filter, an arrangement including an optical circulator, an arrangement including an optical circulator and optical fiber grating, or an arrangement including a fiber coupler.
• The arrangement of the[0056]fiber unit230 included in the dispersion compensating fiber module21 (21A to21C) according to the present invention will be described next with reference to FIGS. 5A, 5B,6, and7.
• FIGS. 5A and 5B are views showing the arrangement of the first embodiment of a fiber unit. A[0057]fiber unit230A shown in FIGS. 5A and 5B can be used as thefiber unit230 of each of the dispersion compensatingfiber modules21A to21C shown in FIGS.2 to4. Thisfiber unit230A has a dispersion compensatingoptical fiber233 and Raman amplificationoptical fiber234 sequentially arranged along a signal light propagation path extending from aninput terminal231A to anoutput terminal232A. End faces of the dispersion compensatingoptical fiber233 and Raman amplificationoptical fiber234 are fusion-spliced to each other. In thefiber unit230A, the signal light input from theinput terminal231A is dispersion-compensated by the dispersion compensatingoptical fiber233, and Raman-amplified by the Raman amplificationoptical fiber234. The resultant signal light is output from theoutput terminal232A. The arrangement shown in FIGS. 5A and 5B is suitable for backward pumping in that the gain of Raman amplification is high because pumping light is input first to the Raman amplificationoptical fiber234. As shown in FIG. 5B, in particular, the arrangement in which anoptical isolator235 is disposed between the dispersion compensatingoptical fiber233 and the Raman amplificationoptical fiber234 is more preferable in that pumping light passing through the Raman amplificationoptical fiber234 does not reach the dispersion compensatingoptical fiber233.
• FIG. 6 is a view showing the arrangement of the second embodiment of the fiber module. A[0058]fiber unit230B shown in FIG. 6 can also be used as thefiber unit230 of each of the dispersion compensatingfiber modules21A to21C shown in FIGS.2 to4. Thisfiber unit230B has a Raman amplificationoptical fiber234 and dispersion compensatingoptical fiber233 sequentially arranged along a signal light propagation path extending from aninput terminal231B to anoutput terminal232B. End faces of the Raman amplificationoptical fiber234 and dispersion compensatingoptical fiber233 are fusion-spliced to each other. In thisfiber unit230B, the signal light input from theinput terminal231B is Raman-amplified by the Raman amplificationoptical fiber234, and dispersion-compensated by the dispersion compensatingoptical fiber233. The resultant signal light is output from theoutput terminal232B. This arrangement is preferable in that an increase in noise figure is suppressed.
• FIG. 7 is a view showing the arrangement of the third embodiment of the fiber module. A[0059]fiber unit230C shown in FIG. 7 can also be used as thefiber unit230 of each of the dispersion compensatingfiber modules21A to21C shown in FIGS.2 to4. Thisfiber unit230C has a dispersion compensatingoptical fiber233a, Raman amplificationoptical fiber234, and dispersion compensatingoptical fiber233bsequentially arranged along a signal light propagation path extending from aninput terminal231C to anoutput terminal232C. End faces of the dispersion compensatingoptical fiber233aand Raman amplificationoptical fiber234 are fusion-spliced to each other. End faces of the Raman amplificationoptical fiber234 and dispersion compensatingoptical fiber233bare fusion-spliced to each other. In thisfiber unit230C, the signal light input from theinput terminal231C is dispersion-compensated by the dispersion compensatingoptical fiber233a, and Raman-amplified by the Raman amplificationoptical fiber234. Thereafter, the light signal is further dispersion-compensated by the dispersion compensatingoptical fiber233b. The resultant signal light is output from theoutput terminal232C. This arrangement is preferable in that the gain of Raman amplification is high and an increase in noise figure is suppressed.
• The dispersion compensating optical fiber[0060]233 (233a,233b) included in the fiber unit230 (230A to230C) has a chromatic dispersion whose sign is different from that of a chromatic dispersion in each ofoptical transmission lines40 and50 of an optical communication system1 (FIG. 1), and compensates for at least the chromatic dispersion of theoptical transmission line40. The dispersion compensatingoptical fiber233 can compensate for the chromatic dispersion in the Raman amplificationoptical fiber234 and can also Raman-amplify signal light.
• The Raman amplification[0061]optical fiber234 included in the fiber unit230 (230A to230C) is an optical fiber with high nonlinearity, and is suitable for Raman amplification of signal light. This Raman amplificationoptical fiber234 has an FOM Raman of 8 (1/dB/W) or more with respect to signal light having a wavelength of 1,650 nm when pumping light having a wavelength of 1,550 nm is supplied, and a chromatic dispersion of 10 ps/nm/km to 50 ps/nm/km in absolute value, preferably 15 ps/nm/km to 40 ps/nm/km in absolute value, and more preferably 20 ps/nm/km to 30 ps/nm/km in absolute value, at a wavelength of 1,650 nm.
• In this case, an FOM Raman is defined by g[0062]_R/A_eff/α_Pwhere g_Ris the Raman gain coefficient, A_effis the effective area, and α_Pis the loss at the pumping light wavelength.
• In addition, as disclosed in Japanese Patent Laid-Open No. 8-248251 ([0063]EP 0 724 171 A2), an effective area (A_eff) is given by$A_{\mathrm{eff}}=2{\pi \left({\int }_0^{\infty }{\mathrm{<figure-callout\; label="E"\; filenames="US20030169986A1-20030911-D00011.png,US20030169986A1-20030911-D00012.png"\; state="\{\{state\}\}">E</figure-callout>}}^2rr\right)}^2/\left({\int }_0^{\infty }{\mathrm{<figure-callout\; label="E"\; filenames="US20030169986A1-20030911-D00011.png,US20030169986A1-20030911-D00012.png"\; state="\{\{state\}\}">E</figure-callout>}}^4rr\right)$

  
• where E is the electric field accompanying propagated light, and r is the distance from the core center in the radial direction.[0064]
• The Raman amplification[0065]optical fiber234 preferably has an effective area A_effof 15 μm²or less at the pumping light wavelength. The Raman amplificationoptical fiber234 preferably has a Raman gain coefficient g_Rof 0.005 (1/Wm) or more. The Raman amplificationoptical fiber234 preferably has a nonlinear coefficient of 20 (1/W/km) or more. In these cases, signal light is effectively Raman-amplified.
• The Raman amplification[0066]optical fiber234 has a chromatic dispersion of 10 ps/nm/km to 50 ps/nm/km in absolute value, preferably 15 ps/nm/km to 40 ps/nm/km in absolute value, and more preferably 20 ps/nm/km to 30 ps/nm/km in absolute value, at a wavelength of 1,650 nm. The Raman amplificationoptical fiber234 preferably has a dispersion slope of −0.3 ps/nm²/km to +0.1 ps/nm²/km at a wavelength of 1,650 nm. In these cases, the dispersion value of thefiber unit230 can be easily adjusted.
• FIG. 8A is a sectional view of an optical fiber (an optical fiber according to the present invention) which can be applied to the Raman amplification[0067]optical fiber234 described above. FIG. 8B shows the refractive index profile of this optical fiber. Anoptical fiber100 has a core region101 (adiameter2aand a maximum refractive index n₁) and a cladding region102 (refractive index n₂) surrounding thecore region101.
• A[0068]refractive index profile150 shown in FIG. 8B corresponds to the refractive index of each portion on a line L in FIG. 8A, aregion151 represents the refractive index of thecore region101 on the line L, and aregion152 represents the refractive index of thecladding region102 on the line L.
• The[0069]optical fiber100 described above is formed by using silica glass (SiO₂) as a main material, doping thecladding region102 with, for example, GeO₂, and doping thecore region101 with, for example, an F element. In addition, thediameter2aof thecore region101 is 4.0 μm. With reference to the refractive index level (no) of pure silica glass, a relative refractive index difference Δ n₁(=(n₁−n₀)/n₀) of thecore region101 is +2.9%, and a relative refractive index difference Δn₂(=(n₂−n₀)/n₀) of thecladding region102 is −0.4%. In this case, theoptical fiber100 has a chromatic dispersion of −15.0 ps/nm/km, at a wavelength of 1,550 nm, a dispersion slope of 0.01 ps/nm²/km, and an effective area of 9.9 μm². Theoptical fiber100 also has a nonlinear coefficient of 22.3 (1/W/km) and a Raman gain coefficient of 0.007 (1/Wm).
• FIG. 9A is a perspective view for explaining a mounted state of the dispersion compensating[0070]optical fiber233 in the dispersion compensating fiber module according to the present invention. FIG. 9B is a perspective view showing a mounted state of an optical component according to the present invention which can be applied to the dispersion compensating fiber module shown in FIG. 9A.
• Referring to FIG. 9A, the dispersion compensating fiber module includes a[0071]case237 and a dispersion compensatingoptical fiber233 which is housed in thecase237 while being wound around abobbin236. Thecase237 has afirst connector237A for receiving signal light from an optical transmission line and asecond connector237B for sending out signal light to the optical transmission line. One end of the dispersion compensatingoptical fiber233 having a chromatic dispersion whose sign is opposite to that of the chromatic dispersion of the optical transmission line is optically coupled to thefirst connector237A. Athird connector233A having a structure that can be coupled to thesecond connector237B is attached to the other end of the dispersion compensatingoptical fiber233.
• If there is no need to perform Raman amplification in this dispersion compensating fiber module, the[0072]third connector233A attached to the other end of the dispersion compensatingoptical fiber233, is directly coupled to thesecond connector237B.
• If Raman amplification is performed in this dispersion compensating fiber module, the optical component shown in FIG. 9B (including an optical fiber according to the present invention as a Raman amplification optical fiber) is disposed between the[0073]second connector237B and thethird connector233A of the dispersion compensatingoptical fiber233. This optical component includes the Raman amplificationoptical fiber234 and anoptical isolator235 having one end optically connected to one end of the Raman amplificationoptical fiber234. In order to allow the optical component to be applied to the dispersion compensating fiber module shown in FIG. 9A, a fourth connector234A is attached to the other end of theoptical isolator235, and afifth connector234B is attached to the other end of the Raman amplificationoptical fiber234. When the dispersion compensatingoptical fiber233 and Raman amplificationoptical fiber234 are to be housed in thecase237, thethird connector233A of the dispersion compensatingoptical fiber233 is directly coupled to the fourth connector of the optical component, and thesecond connector237B of thecase237 is directly coupled to thefifth connector234B of the optical component.
• As described above, both the dispersion compensating[0074]optical fiber233 and Raman amplificationoptical fiber234 included n thefiber unit230 are preferably wound in the form of coils. This is because thefiber unit230 itself can be made compact. When thefiber unit230 constituted by the dispersion compensatingoptical fiber233 and Raman amplificationoptical fiber234 is to be wound around onebobbin236, in particular, it is preferable that the Raman amplificationoptical fiber234 be wound in the form of a coil, and the dispersion compensatingoptical fiber233 be wound around the wound Raman amplificationoptical fiber234 in the form of a coil. This is because the Raman amplificationoptical fiber234 is generally more resistant to a bending loss than the dispersion compensatingoptical fiber233, and hence can be wound into a smaller diameter.
• A plurality of samples of the dispersion compensating[0075]fiber module21 according to the present invention will be described next. Assume that in each prepared sample, signal light of 201 channels at frequency intervals of 50 GHz included in a wavelength band of 1,530 nm to 1,610 nm propagates through theoptical transmission line40 formed from a standard single-mode optical fiber and is input to the dispersion compensatingfiber module21 in therepeater20. FIG. 11 shows a table indicating specifications of dispersion compensating optical fibers and Raman amplification optical fibers which can be applied to the dispersion compensatingfiber module21.
• FIG. 12 is a graph showing the relationship between the required power of pumping light and the length of an optical transmission line in each of dispersion compensating fiber modules as embodiment samples and comparative example samples. FIG. 13 is a graph showing the relationship between the required recovery amount of pumping light power and the length of an optical transmission line in each dispersion compensating fiber module as an embodiment sample with respect to a comparative example sample. In this case, the dispersion compensating fiber module has an arrangement for backward pumping in FIG. 2, and the fiber unit has the arrangement shown in FIG. 5A. In an embodiment sample, the length of a dispersion compensating optical fiber is so set as to compensate for a chromatic dispersion in an optical transmission line, and a Raman amplification optical fiber has a length of 3 km. A comparative example sample includes only a dispersion compensating optical fiber without including any Raman amplification optical fiber.[0076]
• Referring to FIG. 12, a curve G[0077]1110 indicates the relationship between the power of pumping light and the length of an optical transmission line when an effective gain (G) of a dispersion compensating fiber module as an embodiment sample becomes 0 dB; a curve G1120 indicates the same when the effective gain (G) of a dispersion compensating fiber module as an embodiment sample becomes 3 dB; and a curve G1130 indicates the same when the effective gain (G) of a dispersion compensating fiber module as an embodiment sample becomes 6 dB. In addition, a curve G1140 indicates the relationship between the power of pumping light and the length of an optical transmission line when the effective gain (G) of a dispersion compensating fiber module as a comparative example sample becomes 0 dB; a curve G1150 indicates the same when the effective gain (G) of a dispersion compensating fiber module as a comparative example sample becomes 3 dB; and a curve G1160 indicates the same when the effective gain (G) of a dispersion compensating fiber module as a comparative example sample becomes 6 dB. Referring to FIG. 13, a curve G1210 indicates a recovery amount in an embodiment sample with respect to a comparative example sample at a gain of 0 dB; a curve G1220, a recovery amount in an embodiment sample with respect to a comparative example sample at a gain of 3 dB; and a curve G1230, a recovery amount in an embodiment sample with respect to a comparative example sample at a gain of 6 dB.
• As is obvious from FIGS. 12 and 13, as compared with the comparative example samples including only the dispersion compensating optical fibers, in the embodiment samples including the Raman amplification optical fibers in addition to the dispersion compensating optical fibers, the required powers of pumping light were-as low as 100 mW at maximum, and the required recovery amounts of pumping light powers were about 20% to 30%.[0078]
• FIG. 14 is a graph showing NF (Noise Figure) characteristics in forward pumping (curve G[0079]1320) and backward pumping (curve G1310). FIG. 15 is a graph showing RIN (Relative Intensity Noise) characteristics in forward pumping (curve G1410) and backward pumping (curve G1420). Referring to FIG. 15, a curve G1430 indicates a relative intensity noise characteristic without pumping. The arrangement shown in FIG. 3 was used for a dispersion compensating fiber module in the case of forward pumping, whereas the arrangement shown in FIG. 2 was used in the case of backward pumping. In addition, the arrangement shown in FIG. 5A was used for a fiber unit. As is obvious from FIGS. 14 and 15, forward pumping is superior in terms of noise figure, and the influence of relative intensity noise of a pumping light source is smaller in backward pumping.
• FIG. 16 is a graph showing the relationship between the power of pumping light and the effective gain in dispersion compensating fiber modules. FIG. 17 is a graph showing the relationship between the noise figure (NF) and the effective gain in dispersion compensating fiber modules.[0080]
• Referring to FIG. 16, a curve G[0081]1510 indicates the relationship between the power of pumping light and the effective gain in a dispersion compensating fiber module formed from the fiber unit shown in FIG. 5A and has the backward pumping structure shown in FIG. 2; a curve G1520 indicates the same in a dispersion compensating fiber module formed from the fiber unit shown in FIG. 6 and has the backward pumping structure shown in FIG. 2; and a curve G1530 indicates the same in a dispersion compensating fiber module formed from the fiber unit shown in FIG. 7 and has the backward pumping structure shown in FIG. 2. Referring to FIG. 17, a curve G1610 indicates the relationship between the noise figure and the effective gain in the dispersion compensating fiber module formed from the fiber unit shown in FIG. 5A and has the backward pumping structure shown in FIG. 2; a curve G1620 indicates the same in the dispersion compensating fiber module formed from the fiber unit shown in FIG. 6 and has the backward pumping structure shown in FIG. 2; and a curve G1630 indicates the same in the dispersion compensating fiber module formed from the fiber unit shown in FIG. 7 and has the backward pumping structure shown in FIG. 2.
• As is obvious from FIGS. 16 and 17, when a Raman amplification optical fiber is disposed on the output stage of a dispersion compensating optical fiber (FIG. 5A), the required power of pumping light is low, and the pumping efficiency is excellent. When a dispersion compensating optical fiber is disposed on the output stage of a Raman amplification optical fiber (FIG. 6), the resultant arrangement is excellent in terms of noise figure. When a Raman amplification optical fiber is disposed between the first dispersion compensating optical fiber and the second dispersion compensating optical fiber (FIG. 7), the resultant arrangement has intermediate characteristics between those indicated by FIGS. 5A and 6.[0082]
• As has been described above, according to the present invention, since a fiber unit that can be applied to a dispersion compensating fiber module includes an optical fiber that can be used as a Raman amplification optical fiber, signal light is Raman-amplified by the optical fiber, if pumping light for Raman amplification is supplied as needed, as well as being dispersion-compensated. In addition, the signal light is also dispersion-compensated in the dispersion compensating optical fiber included in the dispersion compensating fiber module. With this arrangement, the pumping efficiency in Raman amplification is greatly improved.[0083]

Claims (25)

What is claimed is:

1. An optical fiber having the following characteristics of:

an FOM Raman of 8 (1/dB/W) or more with respect to signal light having a wavelength of 1,650 nm when pumping light having a wavelength of 1,550 nm is supplied; and

a chromatic dispersion having an absolute value of 10 ps/nm/km to 50 ps/nm/km at a wavelength of 1,650 nm.

2. An optical fiber according toclaim 1, wherein the absolute value of the chromatic dispersion at the wavelength of 1,650 nm is 15 ps/nm/km to 40 ps/nm/km.

3. An optical fiber according toclaim 2, wherein the absolute value of the chromatic dispersion at the wavelength of 1,650 nm is 20 ps/nm/km to 30 ps/nm/km.

4. An optical fiber according toclaim 1, further having an effective area of 15 μm²or less with respect to light having the wavelength of 1,550 nm.

5. An optical fiber according toclaim 1, further having a Raman gain coefficient of 0.005 (1/Wm) or more.

6. An optical fiber according toclaim 1, further having a nonlinear coefficient of 20 (1/W/km) or more.

7. An optical fiber according toclaim 1, further having a dispersion slope of −0.3 ps/nm²/km to +0.1 ps/nm²/km at a center wavelength in a signal wavelength band.

8. A dispersion compensating fiber module which compensates for a chromatic dispersion of an optical transmission line through which signal light propagates, comprising:

a dispersion compensating optical fiber having a chromatic dispersion whose sign is opposite to that of the chromatic dispersion of said optical transmission line in a signal wavelength band; and

a Raman amplification optical fiber according toclaim 1.

9. An optical communication system comprising:

an optical transmission line for transmitting signal light; and

a dispersion compensating fiber module according toclaim 8 which performs dispersion compensation of the signal light.

10. A Raman amplifier comprising:

an optical fiber according toclaim 1; and

a supply unit for supplying pumping light to said optical fiber.

11. An amplifier according toclaim 10, further comprising an optical isolator optically connected to one end of said optical fiber.

12. An optical component comprising:

an optical fiber according toclaim 1;

a first optical connector attached to one end of said optical fiber;

an optical isolator having one end optically connected to the other end of said optical fiber; and

a second optical connector attached to the other end of said optical isolator.

13. A component according toclaim 12, further comprising a case which houses said optical fiber and said optical isolator.

14. A dispersion compensating fiber module which compensates for a chromatic dispersion of an optical transmission line through which signal light propagates, comprising:

an input terminal for receiving the signal light;

an output terminal for outputting the signal light;

a dispersion compensating optical fiber disposed on an optical path between said input terminal and said output terminal, said dispersion compensating optical fiber having a chromatic dispersion whose sign is opposite to that of the chromatic dispersion of said optical transmission line in a signal wavelength band; and

a Raman amplification optical fiber disposed on the optical path between said input terminal and said output terminal, said Raman amplification optical fiber having the following characteristics of:

an FOM Raman of 8 (1/dB/W) or more with respect to signal light having a wavelength of 1,650 nm when pumping light having a wavelength of 1,550 nm is supplied, and

a chromatic dispersion having an absolute value of ½ or less that of said dispersion compensating optical fiber at the wavelength of 1,650 nm and 10 ps/nm/km to 50 ps/nm/km.

15. A module according toclaim 14, wherein said Raman amplification optical fiber has the chromatic dispersion with the absolute value of 15 ps/nm/km to 40 ps/nm/km at the wavelength of 1,650 nm.

16. A module according toclaim 14, wherein said Raman amplification optical fiber has the chromatic dispersion with the absolute value of 20 ps/nm/km to 30 ps/nm/km at the wavelength of 1,650 nm.

17. A module according toclaim 14, wherein said Raman amplification optical fiber has an effective area of 15 μm²or less with respect to light having the wavelength of 1,550 nm.

18. A module according toclaim 14, wherein said Raman amplification optical fiber has a Raman gain coefficient of 0.005 (1/Wm) or more.

19. A module according toclaim 14, wherein said Raman amplification optical fiber has a nonlinear coefficient of 20 (1/W/km) or more.

20. A module according toclaim 14, wherein said Raman amplification optical fiber has a dispersion slope of −0.3 ps/nm²/km to +0.1 ps/nm²/km at a center wavelength in a signal wavelength band.

21. A dispersion compensating fiber module for compensating for a chromatic dispersion in an optical transmission line, through which signal light propagates, in a signal wavelength band, comprising:

a case having a first connector for receiving the signal light from the optical transmission line, and a second connector for sending out the signal light to said optical transmission line; and

a dispersion compensating optical fiber which is housed in said case and has a chromatic dispersion whose sign is opposite to that of the chromatic dispersion of said optical transmission line, said dispersion compensating optical fiber having one end optically coupled to said first connector, and the other end to which a third connector having a structure that can be coupled to said second connector is attached.

22. A module according toclaim 21, further comprising a Raman amplification optical fiber housed in said case and having two ends to which fourth and fifth connectors are attached,

said fourth connector attached to one end of said Raman amplification optical fiber being coupled to said third connector attached to the other end of said dispersion compensating optical fiber, and said fifth connector attached to the other end of said Raman amplification optical fiber being coupled to said second connector of said case.

23. A module according toclaim 16, wherein said Raman amplification optical fiber is wound in the form of a coil, and said dispersion compensating optical fiber is wound in the form of a coil on an outer periphery of said Raman amplification optical fiber wound in the form of a coil.

24. An optical communication system comprising:

an optical transmission line for transmitting signal light; and

a dispersion compensating fiber module according toclaim 14 which performs dispersion compensation of the signal light.

25. An optical communication system comprising:

an optical transmission line for transmitting signal light; and

a dispersion compensating fiber module for compensating for a chromatic dispersion in said optical transmission line, said dispersion compensating fiber module including:

a case having a first connector for receiving the signal light from the optical transmission line, and a second connector for sending out the signal light to the optical transmission line, and

a dispersion compensating optical fiber which is housed in said case and has a chromatic dispersion whose sign is opposite to that of the chromatic dispersion of said optical transmission line, said dispersion compensating optical fiber having one end optically coupled to said first connector, and the other end to which a third connector is attached,

said second and third connectors having structures that allow said connectors to be directly coupled to each other and optically connected to each other via a Raman amplification optical fiber.

Images